Why do differences exist among culture regions within a single country?

It has been this way for as long as religion as been around, after a place is conquered people travel to another place to develop and thrive or they practice their faith privately, the Middle East is a prime example of this today people worship god but all in different ways, America is a prime example of this question. The United States is a melting of all the countries around the world so variety is definitely proven and with time comes more ways of travel and cheaper funds to access it.